<p>Set <code>cx</code> attribute of the subelement relatively to the width of the element referenced to by the selector in <code>ref</code> attribute.
    If the value is in the <code>[0, 1]</code> interval (or expressed in percentages, e.g. <code>'80%'</code>), the <code>cx</code> of the subelement will be set as a percentage
    of the width of the referenced element. If the value is <code>&lt;0</code> or <code>&gt;1</code>, the height
    of the subelement will be smaller/bigger than the width of the referenced element by the amount specified.
    Note that this makes sense only for SVG elements that support <code>rx</code> and <code>ry</code> attributes, such
    as <code>&lt;ellipse&gt;</code>.
</p>
